Circuit arrangement including an auxiliary coil for generating a magnetic field periodically varying with time and being adjustable in amplitude

ABSTRACT

A circuit for clamping a magnetic field, such as a television convergence field has a main coil with A.C. current applied to it for generating part of the field. An auxiliary coil is wound on the same core and has applied to it a D.C. current that has a value equal to the average of the A.C. current. Therefore, an extreme value of the resultant field is clamped.

United States Patent Boekhorst l Jan. 30, 1973 [54] CIRCUIT ARRANGEMENT INCLUDING [56] References Cited AN AUXILIARY COIL FOR GENERATING A MAGNETIC FIELD UN'TED STATES PATENTS PERIODICALLY VARYING WITH TIME Ziegel g AND BEING ADJUSTABLE IN I I3 AMPLITUDE 3,258,642 6/1966 Dielz ..3l5/l3 C [75] Inventor: Antonius Boekhorst, Emmasingel, Primary Examiner-Carl D. Quarforth Eindhoven, Netherlands Assistant Examiner-J. M. Potenza [73] Assignee: U.S. Philips Corporation, New Attorney-Frank Tnfa" York 57 ABSTRACT 7 [22] Filed June l9 0 A circuit for clamping a magnetic field, such as a [2]] Appl. No.1 43,369 television convergence field has a main coil with AC. current applied to it for generating part of the field. An auxiliar coil is wound on the same core and has 30 F A l I P I D I Y orelgn pp on y a a applied to it a DC current that has a value equal to June 7, Netherlands the average of the urrent Therefore an ex.

treme value of the resultant field is clamped. [52] U.S. Cl. ..3l5/l3 C A s [51 Int. Cl .1101 j 29/46 14 Claims, 10 Drawing Figures [58] Field of Search ..3l5/l3 C, 13 R, 27 TD, 27 6 DC, 315/22 R ll AAAA PATENTEUJAH30 191s SHEET 10F 2 AGEN'JI' PATENTEDJAH30 I975 SHEET 20F 2 3,714,495

INVENTOR.

ANTONIUS B IK HORST BY ,f/QM Vi A AGE 2T I CIRCUIT ARRANGEMENT INCLUDING AN AUXILIARY COIL FOR GENERATING A MAGNETIC FIELD PERIODICALLY VARYING WITII TIME AND BEING ADJUSTABLE IN AMPLITUDE The invention relates to a circuit arrangement including a coil for generating a magnetic field periodically varying with time and being adjustable in amplitude, which coil is wound on a core of magnetic material through which an alternating current flows.

Such an arrangement may be used in a color picture display device for the purpose of the so-called radial convergence. United Kingdom Pat. specification No. 1,100,151 describes an arrangement wherein the combination of a parabola voltage of line frequency and a sawtooth voltage is applied through a clamping circuit to the input of an amplifier which drives a current through the convergence coil, the convergence coils for the red" and the green electron beams being arranged in parallel. In the same manner the combination of a parabola voltage of field frequency and a sawtooth voltage is likewise applied through a clamping circuit to the input of an amplifier whose input is connected to that of the first-mentioned amplifier. The dynamic convergence may be adjusted by means of potentiometers which are provided at the input of the said amplifiers so that the amplitudes of the parabola currents of line and field frequency and of the sawtooth currents can be adjusted. This is effected after the convergence has been adjusted statically, that is to say, in the center of the displayed picture, by means of permanent magnets. The levels at which the input voltage of each amplifier is clamped are adjustable. After controlling the dynamic convergence the static convergence may be recontrolled if necessary if a shift as a result of the dynamic convergence has taken place.

It is evident that the presence of the said clamping circuits before the amplifiers is only useful when the direct current components are passed on through the amplifiers, that is to say, when the amplifiers and the relevant convergence coils are DC-coupled together. This does not pose a great problem for the convergence of field frequency, for in this case the convergence coil must have a rather high inductance due to the comparatively low frequency ofthe current flowing through this coil.

The alternating current and the direct current component thereof flowing through this coil may therefore be fairly low.

On the other hand, for the convergence of line frequency at which the frequency of the current flowing through the convergence coil is much higher than in the previous case, the inductance of said coil is lower. As a result this current as well as its direct current component are rather high. The power corresponding thereto and being proportional to the square of the current is rather greatand must be provided, by the relevant amplifier which causes quite a considerable dissipation.

In summary, it can be said that the arrangement according to the said United Kingdom patent specification has the following drawbacks. Firstly, the amplifiers and the relevant convergence coils must be DC-coupled, which is not always. possible or desirable. Secondly, the amplifiers for the convergence of line frequency must be able to supply a great power.

The first-mentioned drawback may of course be aliminated by displacing the clampingcircuit to the output of the amplifiers. In such a case, however, the said great power must no longer be dissipated by the amplifiers but by the clamping circuits. The invention is based on the recognition of the fact that the desired clamping action is effected in a different manner and to this end the arrangement according to the invention is characterized in that in order to set an extreme value of the field at a fixed level the current flowing through the coil is provided by two amplifiers substantially adjusted in class B, an auxiliary coil being arranged in series with either of the two amplifiers, a smoothing capacitor being provided so that exclusively a direct current flows through said auxiliary coil, which current is the mean value of the current flowing through the lastmentioned amplifier, the auxiliary coil being wound on the same core and in the same winding sense as the first-mentioned coil and having a higher inductance than the first-mentioned coil, and the ratio of the turns on the two coils being determined by the level at which" the extreme value of the field must be set and by the form as a function of time of the field. If a very special It is to be noted that an arrangement as described in the said United Kingdom Patent Specification may be used without many objections for the convergence in color picture display tubes having a small deflection angle. Powers which were then required were much smaller and therefore passive networks, that is to say, without the use of amplifiers were used. However, the convergence requires a greater power in case of picture display tubes having a greater deflection angle so that the above-mentioned problems become manifest. In color picture display tubes having a deflection angle of, for example, 110 the power required for the horizontal convergence is approximately four times higher than for tubes having a deflection angle of Due to the step according to the invention a considerable amount of power dissipation is eliminated, while it will be shown that the static and dynamic adjustments do not exert influence on each otherQAs a result low power transistors may be used as amplifiers and the different coilsneed not be adjustable, which is a great economy.

It is to be noted that the circuit arrangement according to the invention need not be limited to its use for convergence purposes. In fact, such an arrangement may advantageously be used in all those cases wherein a magnetic field is a periodical function of time and an extreme value ofwhich must be set at a given level.

In order that the invention may be readily carried into effect, a few embodiments thereof will now be described in detail by way of example with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 shows an arrangement according to the invention,

FIG. 2a shows a current waveform which occurs in the arrangement according to FIG. 1 and which is the integral of the waveform shown in FIG. 2b,

FIG. 3 shows an electromagnet which is present in the arrangement according to FIG. 1, and FIG. 4 shows the shape of induction fields generated therein,

FIG. shows a current wave which is the combination of that according to FIG. 2a and a sawtooth wave,

FIG. 6, 7 and 8 show arrangements which are suitable with the current wave according to FIG. 5.

In the embodiment of FIG. 1 the reference numerals 1 and 2 denote input terminals to which parabola voltages of line frequency and pulsatory voltages originating from the line deflection circuit in a color picture display device are applied. Terminal 1 is, for example, a terminal of the capacitor for the so-called S-correction on which a parabola voltage is produced, while terminal 2 may be a point on a winding on the line output transformer. Terminals 1 and 2 are connected by means of a capacitor 3 of low capacitance and resistor 4, capacitor 3 being shunted by the series arrangement of a resistor 5 of fixed value and an adjustable resistor 6. The junction of elements 3, 4 and 6 is connected to earth through a potentiometer 7 and a resistor 8. The wiper on potentiometer 7 is connected to the wiper on a further potentiometer 9 whose ends are connected to earth through two resistors 10 and 10' of equal value. The junction of potentiometer 9 and resistor 10 is connected to the bases of two transistors 11 and 12. These transistors, which are of opposite polarity, constitute a complementary pair, the bases and emitters being interconnected together. Transistor 12, which is of the pnp-type, is shown at the bottom of the Figure. Its collector is connected to a negative supply line V while transistor 11, which is of the npn-type, is.connected through a coil 13 to a positive supply line +V Coil 13 is shunted by means of a resistor 14. Furthermore, the collector of transistor 11 is decoupled for line frequency voltages by means of a capacitor 15 which may be optionally connected to earth or to either of the lines +V,, and V Elements l3, l4 and 15 may alternatively be incorporated in the collector line of transistor 12.

The common point of the emitters of transistors 11 and 12 drives via a capacitor 16 a current through the coil 17 for the horizontal dynamic convergence, the other end of which coil is connected to earth. The series network of a capacitor 18 and a resistor 19 is arranged in parallel with coil 17.

Coils 13 and 17 are wound on the same core of magnetic material and serve for the convergence of the electron beam which must impinge upon red phosphor dots provided on the screen of the picture display tube. In a corresponding manner the junction of potentiometer 9 and resistor 10' is connected to the bases of two transistors 11 and 12 and corresponding elements 13' and 19' inclusive are connected which serve for the green convergence.

The parabola voltage on terminal 1 is differentiated by the network comprising capacitor 3 and resistors 4, 7 and 8, so that a sawtooth voltage increasing with time is produced on the junction of these elements. This voltage is added to the pulsatory voltage originating from the terminal 2 and part of the voltage obtained in this manner drives amplifiers 11, 12 and 11' and 12'.

For further explanation it is now first assumed that the elements l3, l8 and 19 are not present so that the collector of transistor 11 is directly connected to supply voltage +V Since the line repetition frequency is comparatively high, the resistance of coil 17 is negligible. Consequently, a current i flows through the coil which current, as a function of time, is the integral of the control voltage of the amplifier and is thus the combination of a sawtooth current and a parabola current. In a corresponding manner a current of the same form flows through coil 17. As is known this form is desired in order to effect a satisfactory horizontal convergence everywhere on the screen of the picture display tube.

For practical reasons a parabola voltage on terminal 1 has been taken as a starting point because there is no sawtooth voltage proportional to the line deflection current available in the display device, which sawtooth voltage could have been applied to the input of the circuit arrangement so that the differentiating network 3, 4 could have been omitted. As will be further explained said proportionality is necessary.

The amplitude of the pulsatory voltage which is applied to both amplifiers can be adjusted by means of variable resistor 6. Variable resistor 6 therefore ensures the adjustment of the amplitude of the sawtooth convergence current. By displacing the wiper on potentiometer 7 the amplitude of the combined control voltage is adjusted. If the wiper on potentiometer 9 is displaced, the combined current which flows through one convergence coil is increased while the current flowing through the other coil is decreased by the same value. The amplitudes of the convergence currents for red and for green may therefore be varied by means of potentiometer 7 in the same direction and by means of potentiometer 9 in the opposite direction so that the convergence can be controlled satisfactorily in a simple manner.

Since no direct voltages are applied to the terminals 1 and 2 and since the bases of transistors 11, 12, 11', 12 are at earth potential and since both amplifiers are fed by two supply voltages +V,, and V,,, of equal absolute value of, for example, +2OV and 20V, these amplifiers automatically function in class B.

If two supply voltages of equal absolute value are not available, that is to say, if only one supply voltage of, for example, +4OV is present in the display device, the ends of resistors 10 and 10' connected to earth in FIG. 1 must be applied to a smoothed voltage of, for example, +2OV and the input voltages on terminals 1 and 2 must be applied through capacitors to the rest of the circuit arrangement. Due to this class B-adjustment the convergence current is alternately provided by transistors 11 and 11' and by transistors 12 and 12, respectively. Due to the class B-adjustment the convergence current is proportional to the control voltage of transistors 11 and 12 and 11' and 12', respectively. These transistors may alternatively be adjusted in class AB in order to compensate the non-linearity of the characteristic curves thereof.

FIG. 2a shows the waveform as a function of time of the current i which flows through one of the convergence coils, for example, coil 17, it being assumed that there is no sawtooth component. Since the current i flowing through the coil cannot include a direct current component, the current waveform shown in FIG. 2a is situated around the zero line in such a manner that the mean value of the current is zero, the positive part being provided by transistor 11 and the negative part being provided by transistor 12. Current i of FIG. 2a is, as a function of time, the integral of the sawtooth function of FIG. 2b, the interval 2, t H being a full line period. The peak-to-peak value of the current flowing during the scan period and corresponding to a given position of the control members described is referred to as I. The peak value above the value indicated by I follows from the ratio of the flyback period relative to the line period of the waveform shown in FIG. 2b and may hereinafter be left out of consideration. It is then possible to calculate the mean value I, of current i as a function of I. In fact, the mathematical expressions of the two parabola parts of FIG. 2a can be derived with the aid of the sawtooth function of FIG. 2b. The origin of the time axis is chosen at the instant when current i has reached its negative maximum. The result of the calculation is that the mean value of the overall current i is equal to [/2 if percent of the overall line period has been chosen for the flyback period. The negative maximum occurs in the middle of the line scan period, that is to say, at an instant when only the static convergence should be active. In other words, this maximum should be at zero. If in addition, the peak-to-peak value I of the parabola current flowing during the scan period varies, the negative maximum I/2 likewise varies, which means that the convergence varies in the middle of all horizontal lines when the convergence is adjusted on either side of these lines. It is therefore desirable to adjust the said negative maximum at a fixed level in some manner or other, which was also the purpose in the United Kingdom patent specification referred to. In addition this level must be zero in order that the field generated by current i in the convergence coil is zero in the middle of the horizontal lines, the convergence being adjusted statically so that the static and dynamic convergence are independent of each other.

In color picture display tubes having a deflection angle of 110 it is possible that the peak-to-peak amplitude I is, for example, 300 mA. This value is determined by the inductance of coil 17 which in turn is determined by the admissible voltage which the transistors can stand and by the value of the supply voltages. A clamping circuit provided behind capacitor 16 should therefore be able to provide [/2 150 mA. Thus, if such clamping circuit is arranged in the base line of transistors 11 and 12 and if convergence coil 17 is D.C. connected to the junction of the emitters of these transistors, in which case the direct voltage levels must of course be adapted to this new situation, the said transistors must be able to provide this amount of 150 mA. This means that the power taken from the supply must amount to 0.15 X 40 6 W for the direct current component alone. The object of the step according to the invention is to reduce this considerable and useless loss ofpower.

FIG. 3 diagrammatically shows an electromagnet by means of which the red convergence is established. Coils 13 and 17 are wound on a U-shaped core of magnetic material, coil 13 having more turns than coil 17. The coil 20 for the vertical convergence is wound on the same core, which core is, however, associated with a circuit arrangement other than the relevant one. windings l3 and 17 are wound in such a sense that an increase of the induction field which is generated by the current flowing in one winding corresponds to an increase of the field which is generated by the current flowing in the other winding. Since an induction field is proportional to the current which flows through the relevant coil, the field generated by coil 17 is the combination of a sawtooth and a parabola field, while the field generated by coil 13 is a direct current field.

Again assuming the current flowing through coil 17 to be purely parabolic, the induction field generated by this coil assumes the shape which is shown by curve B in FIG. 4 and wherein field B, becomes negative and reaches a maximum negative value around the middle of one line scan period. However, the direct current which flows through coil 13 is equal to the mean value of the pulsatory current flowing through transistor 11, that is to say, every time between the instants t and t t and t, etc. in FIG. 2a. This mean value is of course equal to the mean value of the current which flows through transistor 12 between the instants t, and etc. in FIG. 2a. This mean value t can be calculated by means of the simple waveform referred to in this case. It is found that the ratio i li between the absolute value-of the maximum current i,,,,,, and direct current i as a function of the flyback ratio is the following:

With z=20 percent it is found that i0 is less than in, =71 2 b '5 factor which is eqaaffaaapimmatwfis; Consequently, if the ratio of the turns on coils 13 and 17 is equal to approximately 2.65, coil 13 produces an induction field B2 which is positive due to the abo ve mentioned winding serge of both coils and which is equal to the absolute value of th rfiafiti Ii um ofTrfiiliiiori d'ieId BT TITresultant induction field which is produced in the electromagnet is shown by B3 in FIG. 4. Direct current field B compensates to the negative maximum value of alternating current field B, so that the extreme value of field B is zero. If the amplitude of the parabola current is adjusted to a higher value, so that the induction field B becomes larger, the negative maximum thereof increases as well. However, in that case the direct current flowing through the transistors 11 and 12 increases proportionally thereto and the extreme value of the then resultant induction field remains zero. The desired clamping action is therefore effected and is independent of the amplitude of the convergence current, but is only dependent on the waveform and on the clamping level, in this case zero. Once the static convergence is adjusted by means of, for example, a permanent magnet, it does not substantially vary thereafter during the control of the dynamic convergence. A condition for a satisfactory operation of the arrangement is that the used magnetic material does not have any substantial hysteresis and that the induc tion field and the signals used are always proportional to the magnetic field. Otherwise distortion would occur.

It is to be noted that if the control signal were reversed relative to the described signal, the winding sense of coil 13 should be reversed because the maximum of induction field B would then be positive. A considerable advantage of the arrangement according to the invention is that both coils 13 and 17 need not be adjusted. In fact, once the ratio of the turns on both windings is chosen, the resultant field B, is always clamped against zero as is shown in FIG. 4. However, to take account of the tolerances in the different parts of the picture display device, the precaution can be taken to choose the said ratio to be somewhat higher than is necessary so that an overcompensation would be created, and to arrange a damping resistor 14 in parallel with coil 13 by which the correct compensation can be achieved. The value of this resistor is not critical so that it may have a fixed value. This is only possible because the resistance of coil 13 is not negligibly low, but is in the order of I Ohms. Thus a distribution of the direct current flowing through transistors 11 and 12 occurs between coil 13 and damping resistor 14 so that the number of turns on coil 13 must be more than the number calculated above.

The clamping action described is draws only a small amount of power and this may be explained as follows. If the peak-to-peak amplitude of the convergence current is 300 mA, the negative maximum value thereof is 150 mA if this current is purely parabolic. If the direct current flowing through transistors 11 and 12 is, for example, 3 times smaller than the negative maximum value of the current flowing through coil 17, that is to say if this direct current has an intensity of approximately 50 mA, then the power taken from the supply is only 0.05 X 40 2 W due to the direct current component. This is a considerable economy relative to the previously found Figure of 6 W and hence transistors may be used for transistors 11 and 12 which are suitable for lower powers, which may be considered to be an advantage of the arrangement according to the present invention.

The series network of capacitor 18 and the resistor 19 of relatively low value, which is arranged in parallel with coil 17, has for its object to add a pulsatory voltage to the parabola current flowing though transistors 11 and 12, which causes a further reduction of the dissipation in the transistors. A similar result may alternatively be achieved with the aid of series arrangements of a coil and a capacitor.

It is to be noted that the voltages on terminals 1 and 2 are proportional to the deflection current flowing through the line deflection coils. This current is not constant for each line, but varies as a result of, for example, the correction of the so-called East-West pin cushion distortion. It is alternatively possible for this line deflection current to vary as a result of variations in the mains voltage. A known step of stabilizing the width of the displayed picture in this case is done by causing the EHT generated in the line deflection circuit and required for the final anode of the picture display tube to vary by the same percentage as the mains voltage and to cause the deflection current to vary by half the percentage. It is evident that the variation in the convergence current must be proportional to that of the deflection current if the convergence is to be satisfactory in any area on the screen of the picture display tube.

The arrangement according to FIG. 1 functions as desired when the current flowing through coils 17 is purely parabolic or when at least the sawtooth component is small. However, if this component is rendered comparatively large with the aid of resistor 6, the ratio of the inductance of coils 13 and 17 no longer corresponds to the new mean value i of the current which now flows through transistors 11 and 12. FIG. 5 shows the broken line curve representing the sum of a parabola waveform and a sawtooth waveform increasing with time, the parabola component being the same as that in FIG. 2a. FIG. 5 shows that the new mean value i It is true that the zero axis is somewhat displaced, but the area of the positive portion has become larger because the amplitude at the instant t end of the scan) is higher and because the instant at which the zero axis is intersected has been displaced to the instant t to that transistor 11 conducts over a longer period than in the case of the solid line parabola waveform.

It is to be noted that, as is evident from FIG. 5, the maximum negative value does not occur in the middle of the scan period but occurs earlier. If clamping were effected in known manner, a convergence error would always occur in the middle which would be dependent on the amplitude of the sawtooth current component. Similar reasons apply if the sawtooth waveform decreases with time or if coil 13 is incorporated in the collector line of transistor 12.

FIG. 6a shows in a simplified form an arrangement for the convergence of one electron gun by which the effect described is compensated. Positive and negative pulses are applied to the ends of a linear potentiometer 6' which pulses originate, for example, from a winding 21 of the line output transformer and thus a pulsatory voltage is derived from the wiper thereof, which voltage is variable in both amplitude and polarity. This voltage is added across resistor 10 to a sawtooth voltage originating from potentiometer 7 and the voltage thus obtained is applied to the bases of transistors 11 and 12. Both ends ofa linear potentiometer 22 are connected to a positive auxiliary supply voltage which is higher than +V while the wiper thereof is connected through a resistor 23 to the collector of transistor 11. The wipers on both potentiometers 6' and 22 are coupled mechanically. If both wipers are positioned at one end of the respective potentiometers, the sawtooth current component flowing through coil 17 becomes maximum, while the direct current originating from the auxiliary supply voltage and flowing through the parallel arrangement of coil 13 and resistor 14 is then at a maximum, namely in the direction opposite to that of the collector current of transistor 11. This direct current is at a minimum when both wipers are positioned in the middle, that is to say, when there does not flow any sawtooth current component. Values may be chosen for the auxiliary supply voltage and for resistors 14, 22 and 23 such that the said direct current through coil 13 is substantially proportional to the amplitude adjusted with the aid of potentiometer 6, of the sawtooth current component flowing through coil 17. In this manner the resultant direct current through coil 13 remains substantially independent of the sawtooth current component. FIG. 6b shows a modification wherein only one potentiometer 6' is used. In this case winding 21 has a central tapping which is connected to the auxiliary supply voltage.

In the embodiments of FIG. 7a and 7b an additional compensation coil 24 is used, while no additional supply voltage is necessary. Coil 24 is'wound on the same core as coils 13 and 17 and a direct current flows therethrough which is substantially proportional to the magnitude of the sawtooth current component through coil 17 by means of either two mechanically coupled linear potentiometers (FIG. 7a) or a single potentiometer (FIG. 7b). The winding sense of coil 24 in both cases is such that the induction field generated thereby is reduced relative to that of coil 13. The end of coil 24 not connected to earth is decoupled for the line frequency in the Example of FIG. 7b by means of a capacitor. If the different parts of the circuit arrangement are not sufficiently separated from one another, coil 24 may be driven by a separation transistor. The coil for the vertical convergence may be used both for coil 24 and for coil 13. For the said separation transistor which must drive a direct current though coil 24 a transistor ensuring the alternating current amplification for the vertical convergence may be used.

FIG. 8 shows a further embodiment. In this embodiment the damping resistor 14 varies as a function of the magnitude of the sawtooth current component through coil 17. For this purpose resistor 14 is formed as the series arrangement of a resistor 14 of fixed value and a linear potentiometer 14" whose wiper is mechanically coupled to that of potentiometer 6' and both ends of which are connected to resister 14', while a resistor is incorporated in series with coil 13. When both wipers are positioned in the middle, a sawtooth current component does not flow through coil 17 while the direct current through coil 13 is at a maximum, As soon as the sawtooth current component does flow through coil 17, the direct current through coil 13 is reduced to substantially the same extent, which is desirable. If R and R are referred to as the values of resistors 14' and of the total resistance of potentiometer 17", respectively, if x is the value of the resistor situated between the wiper thereof and its lower end in FIG. 8 and if R is the resistance of the series arrangement of coil 13 and resistor 25, the current i flowing through coil 13 then is:

wherein i is the collector current of transistor 11. Resistance x is proportional to the angle of rotation of potentiometers 14" and 6'. The current flowing through coil 13 is therefore a function of angle 4: with a maximum for d) maxl2 wherein 4), is the maximum of 4), that is to say, for the angle at which no pulsatory control voltage is applied to transistors 11 and 12. Resistor 14, potentiometer 14" and resistor 25 a voltages so that the current through coil 13 varies little. However, since the resistance of the series arrangement of resistor 14' and 14" of FIG. 8 is of the same order as that of the resistive part of the impedance of coil 13, a

temperature variation will result in a different current distribution so that the current flowing through coil 13 will vary For this reason, a thermistor having a negative temperature coefficient (NTC) may be chosen for resistor 25 in the embodiment according to FIG. 8, so that said variation may be compensated to a sufficient extent. The arrangements described ensure the horizontal convergence of the green and red electron beams. However, it will be evident that similar arrangements may alternatively be used for the blue or for the vertical convergence. For the latter case, however, the relevant coil must have greater dimensions since the field repetition frequency is much smaller than the line repetition frequency.

In the embodiments described transistors are used as amplifier elements. The principle of the invention is not affected if different known amplifier elements are used such as, for example, electron valves or field effect transistors. It is alternatively possible to use a circuit arrangement according to the invention wherein waveforms for the magnetic field are used which are different from the substantially parabolic field described. For example, if the field is sawtooth-shaped whose negative minimum must be adjusted at a certain level, the auxiliary coil through which the direct current of the transistors flows must have a number of turns which is four times higher than that of the coil generating the alternating field if no damping resistor 14 is present.

Finally it is to be noted that the field-of application of the circuit arrangement according to the invention need not be limited to convergence circuits but may be used in all cases where an extreme value of periodically varying magnetic field must be adjusted at a certain level.

What is claimed is:

l. A circuit comprising a magnetic core; a main coil wound on said core; means for providing an alternating current to said coil comprising a pair of linear amplifiers coupled thereto, whereby said coilgenerates a time varying magnetic field; and means for clamping an extreme value of said field to a selected level comprising a first auxiliary coil wound on said core in the same sense as said main coil and being coupled in series with one of said amplifiers, and a capacitor coupled in parallel with said auxiliary coil, whereby only direct current flows there through, the value of said direct current being the mean value of said alternating current, said auxiliary coil having a higher inductance than said main coil, the turns ratio therebet-ween being a function of said level and the waveform of said time varying field.

2. A circuit as claimed in claim 1 wherein said main coil field varies in accordance with a parabola function and said ratio is between 2.5 and 3.

3. A circuit as claimed in claim 1 wherein said auxiliary coil comprises an internal resistance and said main coil field varies in accordance with a parabola function and further comprising a damping resistor coupled in parallel with said auxiliary coil, whereby a reduction in the current therethrough is caused, and 2.5n S r 5 3n, wherein r equals said ratio, and n equals the amount of said reduction.

4. A circuit as claimed in claim 1 wherein said main coil field varies in accordance with the sum of parabola and sawtooth functions, and further comprising means for applying a compensating direct current through said auxiliary coil in a direction opposite to that of first recited direct current and having an amplitude approximately proportional to the amplitude of said sawtooth function current.

5. A circuit as claimed in claim 4 further comprising a pair of mechanically coupled linear potentiometer means for adjusting the amplitudes of said parabola and compensating currents respectively.

6. A circuit as claimed in claim 1 wherein said main coil field varies in accordance with the sum of parabola and sawtooth functions and further comprising a second auxiliary coil mounted on said core and means for applying to said second auxiliary coil a direct current having a magnitude approximately proportional to the amplitude of said sawtooth function current and a direction wherein the magnetic field produced by said second auxiliary coil is in the opposite direction from that produced by said first auxiliary coil.

7. A circuit as claimed in claim 6 further comprising a pair of linear mechanically coupled potentiometer means for adjusting the amplitudes of the currents in said second auxiliary coil and said sawtooth current.

8. A circuit as claimed in claim 1 wherein said auxiliary coil comprises an internal resistance, said main coil field varies in accordance with the sum of a sawtooth and parabola function, and further comprising a negative temperature coefficient thermistor coupled in series with said auxiliary coil, a series circuit including a fixed and a variable resistor, said series circuit being coupled in parallel with said auxiliary coil and thermistor, and a potentiometer means mechanically coupled to said variable resistor for adjusting the amplitude of said sawtooth function current.

9. A circuit as claimed in claim 1 further comprising a series circuit including a low value resistor and a capacitor, said series circuit being parallel coupled to said main coil.

10. A circuit as claimed in claim 1 further comprising a series circuit including a coil and a capacitor, said series circuit being parallel coupled to said main coil.

11. A circuit as claimed in claim 1 wherein said main coil comprises a radial convergence coil of a color cathode ray tube for converging one of the electron beams therein.

12. A circuit as claimed in claim 11 wherein said main coil comprises a horizontal convergence coil.

13. A circuit as claimed in claim 1 wherein said amplifiers comprise class B amplifiers.

14. A circuit as claimed in claim 1 wherein said amplifiers comprise class AB amplifiers.

I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,714,495 Dated January 30, 197.3

Inventor(s) Antonlus ISt It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In The Title Page below "June 7, 1969" insert March 7, 1970 below "6908715" insert 7003282 FORM P0-105O (10-69) uscoMM-Dc 60376-P69 U45. GOVERNMENT PRINTING OFFICE I969 0-366-33L UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. 3,714,495 January 30, 1973 Dated Pa e 2 lnventol-( Antonius Boekhorst g It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Add Figures 58 as shown in the accompanying sheets I T I \i I I I W; A; I /ffi no w a! lLili I 0 "w T l I if I I I \QIF/ l I H l h k ORM PO-IOSO (10-69) USCOMM-DC 0O376-P69 U45, GOVERNMENT PRINTING OFFICE: I'll OJ66-334.

I C Page 3 Patent No. 3,714,495

'VWW

U whim I. M 0 F5 m w 3 N a F 2 M a... m. mi 1...; h L. W J m w n a m w y 3,714,495 Page 5 Signed and sealed this 5th day of November 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents 

1. A circuit comprising a magnetic core; a main coil wound on said core; means for providing an alternating current to said coil comprising a pair of linear amplifiers coupled thereto, whereby said coil generates a time varying magnetic field; and means for clamping an extreme value of said field to a selected level comprising a first auxiliary coil wound on said core in the same sense as said main coil and being coupled in series with one of said amplifiers, and a capacitor coupled in parallel with said auxiliary coil, whereby only direct current flows there through, the value of said direct current being the mean value of said alternating current, said auxiliary coil having a higher inductance than said main coil, the turns ratio therebetween being a function of said level and the waveform of said time varying field.
 1. A circuit comprising a magnetic core; a main coil wound on said core; means for providing an alternating current to said coil comprising a pair of linear amplifiers coupled thereto, whereby said coil generates a time varying magnetic field; and means for clamping an extreme value of said field to a selected level comprising a first auxiliary coil wound on said core in the same sense as said main coil and being coupled in series with one of said amplifiers, and a capacitor coupled in parallel with said auxiliary coil, whereby only direct current flows there through, the value of said direct current being the mean value of said alternating current, said auxiliary coil having a higher inductance than said main coil, the turns ratio therebetween being a function of said level and the waveform of said time varying field.
 2. A circuit as claimed in claim 1 wherein said main coil field varies in accordance with a parabola function and said ratio is between 2.5 and
 3. 3. A circuit as claimed in claim 1 wherein said auxiliary coil comprises an internal resistance and said main coil field varies in accordance with a parabola function and further comprising a damping resistor coupled in parallel with said auxiliary coil, whereby a reduction in the current therethrough is caused, and 2.5n < or = r < or = 3n, wherein r equals said ratio, and n equals the amount of said reduction.
 4. A circuit as claimed in claim 1 wherein said main coil field varies in accordance with the sum of parabola and sawtooth functions, and further comprising means for applying a compensating direct current through said auxiliary coil in a direction opposite to that of first recited direct current and having an amplitude approximately proportional to the amplitude of said sawtooth function current.
 5. A circuit as claimed in claim 4 further comprising a pair of mechanically coupled linear potentiometer means for adjusting the amplitudes of said parabola and compensating currents respectively.
 6. A circuit as claimed in claim 1 wherein said main coil field varies in accordance with the sum of parabola and sawtooth functions and further comprising a second auxiliary coil mounted on said core and means for applying to said second auxiliary coil a direct current having a magnitude approximately proportional to the amplitude of said sawtooth function current and a direction wherein the magnetic field produced by said second auxiliary coil is in the opposite direction from that produced by said first auxiliary coil.
 7. A circuit as claimed in claim 6 further comprising a pair of linear mechanically coupled potentiometer means for adjusting the amplitudes of the currents in said second auxiliary coil and said sawtooth current.
 8. A circuit as claimed in claim 1 wherein said auxiliary coil comprises an internal resistance, said main coil field varies in accordance with the sum of a sawtooth and parabola function, and further comprising a negative temperature coefficient thermistor coupled in series with said auxiliary coil, a series circuit including a fixed and a variable resistor, said series circuit being coupled in parallel with said auxiliary coil and thermistor, and a potentiometer means mechanically coupled to said variable resistor for adjusting the amplitude of said sawtooth function current.
 9. A circuit as claimed in claim 1 further comprising a series circuit including a low value resistor and a capacitor, said series circuit being parallel coupled to said main coil.
 10. A circuit as claimed in claim 1 further comprising a series circuit including a coil and a capacitor, said series circuit being parallel coupled to said main coil.
 11. A circuit as claimed in claim 1 wherein said main coil comprises a radial convergence coil of a color cathode ray tube for converging one of the electron beams therein.
 12. A circuit as claimed in claim 11 wherein said main coil comprises a horizontal convergence coil.
 13. A circuit as claimed in claim 1 wherein said amplifiers comprise class B amplifiers. 